The agent's shot is high and to the left, appearing as a blue dot on the projection screen. The simulated encounter is over, and Ricardo Gonzalez wipes sweat from his forehead. "I did not want to underestimate her," the trainee tells the superviser administering the test.

At the Federal Law Enforcement Training Center (FLETC) in Artesia, N.M., 200 miles from the border with Mexico, newly minted border patrol agents take their final exam on the use of lethal force. It's a part of border security that has lately been a bone of contention between agents in the field and top managers, especially now that border violence is on the rise and US agents are at greater risk of coming under attack.

"A lot of agents on the border right now feel a heightened sense of anxiety because of the increase in violence," says David Shirk, director of the Trans-Border Institute at the University of San Diego. They "are encountering heavily armed and very dangerous criminals. and they may be a little more trigger-happy."

At the center of the storm over use of force are two border patrol agents in west Texas who, in January, began serving prison time for shooting and wounding a drug smuggler as he fled toward Mexico and then covering up evidence. Their case has become a cause célèbre for ardent proponents of a border crackdown, conservative bloggers, and some lawmakers in Congress – there was even a petition to the White House to prevent the agents' incarceration.

But for field agents, the case goes deeper, chilling both their willingness to use deadly force and their morale, says T.J. Bonner, president of the National Border Patrol Council, the union that represents 11,000 nonsupervisory personnel at the agency. A more recent case of a border patrol agent in Arizona, charged in April with the first-degree murder of a Mexican national, has the potential to intensify agents' misgivings, he says.

The union leader, an agent based in San Diego, explained in a recent phone interview the rank-and-file's main concern: " 'If I fire at this guy, am I going to have the backing of the guys above me?' That split second can mean the difference between life and death."

Agents are now more likely to avoid situations in which they might have to use deadly force, adds Mr. Bonner. "There's a very real fear about going the extra mile. That's a clear sentiment."

Agents' discontent coincides with agency efforts to add 6,000 personnel to the border patrol – and also with a spike in assaults on agents along parts of the border.

Nationwide, attacks on agents since October rose 3 percent over the same period a year earlier. But in the Yuma sector in western Arizona, a hotbed of smuggling activity, they've jumped 56 percent. Officials say the new level of violence is proof that the border patrol's beefed-up presence – a pillar of the Bush administration's plan to secure the border – is making smugglers more desperate.

In this context, Bonner says, morale among field agents has been deteriorating – and that the case of the jailed agents, José Compean and Ignacio Ramos, is a "big factor." In February – after the two began serving 12- and 11-year sentences, respectively – the union leadership unanimously voted "no confidence" in border patrol chief David Aguilar, the first such resolution in the union's 42-year history. In listing its grievances, the union cited the Ramos and Compean case, faulting the agency and government for "believing the perjured allegations of criminals over the sworn testimony of innocent border patrol agents." The union's position is that the agents were guilty of "administrative missteps," not a crime.

To the US prosecutor who tried the case, the missteps were the most telling clues of wrongdoing. "The damning piece of evidence is they shot 15 times and tried to cover it up," says US attorney Johnny Sutton of the 2005 incident. The circumstances of the case are these: On the afternoon of Feb. 17, 2005, Mr. Ramos and Mr. Compean were alerted that a van heading north into Fabens, Texas, had just fled an attempted traffic stop by a border patrol agent. The two joined the pursuit of Osvaldo Aldrete Davila, who had turned the van – which was loaded with marijuana – back toward Mexico. Mr. Davila abandoned the van when it became stuck in the dirt and was apprehended by Compean, at gunpoint, in a ditch between Mexico and the US. Accounts differ about whether there was a scuffle, but Davila was running into Mexico when Ramos and Compean saw a shiny black object in his hand, according to their testimony. The agents fired 15 shots at the retreating smuggler, hitting him once. Davila escaped, and claimed later in court that he was not armed. The agents picked up the empty shells and did not give an oral report, required whenever an agent discharges a weapon.The two agents were convicted in March 2006 by a federal jury in El Paso. Davila testified against them, receiving immunity from prosecution for drug smuggling.

Mr. Sutton disputes any claim that the government isn't supportive of law-enforcement officers. "Some people try to imply we don't back up border patrol agents.... We do back them up ... when they come forward and explain in good faith what they did." Sutton notes 14 cases in his west Texas district over the past nine years in which border agents who shot suspects – killing them in four incidents – were cleared after explaining the need for lethal force.

Justification for shooting to kill, say officials, is an agent's perception of imminent danger. "You shoot because you believe your life is in danger, or the life of an innocent third party," says Kevin McMichael, a superviser of firearms training at FLETC, where trainees spend 16 weeks. Recruits are trained to react subconsciously, he says. "But you're still going to be judged on anything you do."They are also taught to gauge a suspect's resistance – from compliant to passive to active – in deciding on an appropriate level of force. When a suspect with a gun in his belt wheels suddenly, or when a woman brandishes an object from her purse, agents must assess how their own physical strength stacks up against the suspect's.

Though rising violence on the border can also influence an agent's decision to use force, it does not necessarily follow that the agency's policy needs revising, says Armand Peschard-Sverdrup, director of the Mexico Project at the Center for Strategic and International Studies in Washington. Cases of agents misusing deadly force are too few to suggest that the policy is flawed, he says."You're going to end up having incidents ... given the conditions, the frequency of crossing, and the wide range of individuals border patrol agents come across," says Mr. Peschard-Sverdrup. But "there haven't been enough incidents to warrant a change."

Even if policing the border is riskier now, Mr. Gonzalez, the trainee at FLETC, is OK with that. He will join the Laredo sector in Texas when he graduates from the academy.

"It's a secure job," he says, explaining his motivation to become an agent. "Right now, ... we need to enforce [the border] more than ever. Plus, I like the action."